The present invention relates to electrical contacts, and particularly to electrical contacts having a compliant section for press-fitting into, e.g., round, plated through holes in printed circuit boards.
Compliant press-fit electrical contacts are advantageous for printed circuit board applications, since they make solderless, yet electrically sound, connections. One major problem with these contacts is that the compliant section must adapt to a wide variation in hole sizes, since it is difficult to manufacture printed circuit board holes which have extremely tight tolerances. For example, the industry tolerance standard for a 0.040 inch diameter finished hole size is .+-.0.003 inch, yielding a hole size range from 0.037 diameter to 0.043 diameter. After assembly into any hole within the tolerance range, the contact must withstand an axial load (i.e., withdrawal force) of ten pounds without displacement.
While many prior art designs provide the required ten pound withdrawal (or push-out) force, most require very high assembly (push-in) forces, especially in the smaller 0.037 inch diameter holes. This increase in assembly forces for the smaller holes can become quite significant when, e.g., 100 or more contacts are assembled at one time. Furthermore, some contacts tend to damage the hole and adjacent substrate during assembly, thereby rendering it unsatisfactory for installation of a replacement contact.
Two types of prior art compliant press-fit contacts are commonly available, namely the crescent type and the split beam type. The crescent type is structurally similar to the well known roll-pin used in many mechanical design applications in that it is substantially cylindrical in shape with a longitudinal opening of sufficient width to permit a predetermined reduction in its apparent diameter when laterally constricted. This crescent type contact has a generally uniform C-shaped cross section, and has smooth, continuous inner and outer surfaces. As the contact is pressed into a hole, its arms are deflected inwardly in a spring-like manner to provide the necessary interference fit. Due to the uniformity of cross section, deformation occurs throughout most of the cross sectional area of the contact during insertion, and therefore, whether such deformation is plastic or elastic or a combination thereof, the force required for additional deflection, when the contact is pressed into progressively smaller holes, increases at a relatively high, generally constant rate. Thus, as with many prior art contacts, this type of contact typically requires a substantially larger push-in force for the smaller 0.037 inch diameter hole than for the larger 0.043 inch diameter hole.
The split beam type contact is similar in structure to the eye portion of a needle. As the contact is pressed into a hole, the split beams deflect towards each other to close the "eye" therebetween. This contact is fundamentally different from the crescent type contact in that it collapses in a single plane passing longitudinally through the split beams. The crescent type contact, on the other hand, does not collapse in a single plane, but instead along the circumference of the contact. That is, when viewed cross sectionally, the crescent shaped arms slide circumferentially along the inner peripheral surface of the hole during closure, in contrast to the split beams which remain essentially stationary with respect to the inner peripheral surface of the hole during closure, and close without such circumferential sliding. Accordingly, contacts such as the split beam type contact will be referred to as "planar collapsible," while contacts, such as the crescent type contact will be referred to as "circumferentially collapsible."
Another problem with prior art compliant sections is that they are typically difficult to manufacture. The split beam design typically requires fragile, delicate punches, while the crescent shape design usually involves rolling operations or complex multi-station rounding operations. Such manufacturing problems make miniaturization of the prior art contacts difficult.